You can use EITHER 12AU7 type tubes or 12BH7 tubes as driver tubes in any of the VTA amps. Some people like 12BH7 tubes because they are a little taller than a 12AU7 - maybe the "bigger is better" theory in action here ... Actually I now feel that there isn't a whole lot of difference in sound quality between the 12AU7 type tube or a 12BH7 in these amps. Right now I am using a Mullard 12AU7 in the front position and a GE 12BH7 in the rear position of my own M-125 amps. I like the way the amps sound set up this way at least in my own music system. You have to realize that different people have different ways that they want their music system to sound. The nice thing about 12AU7's is that there are still many USA made 12AU7's and military/industrial 12AU7 types (5963, 5814 and 6189) still in existence. You can many times pick up from Ebay and other online internet tube vendors quality USA made (RCA, GE and Sylvania) versions of this tube at reasonable prices.
Most readers of this forum now know that the FRONT tube on the VTA M-125's and the CENTER tube on the VTA ST-70/ST-120 is the single tube that you can change out that will usually show some sound difference in the amp(s).
Capacitive discharge does occur when you shut the amps OFF. The components within the amp will look at progressively lower operating voltages. Sometimes you may get a funny noise from a tube when it is faced with an operating voltage well below what the tube is supposed to see. This is not harmful to the amp ..
Just to reconfirm that on VTA120, I can change the Centre tube to 12BH7 while the L&R will use 12AU7, and this will be applicable as well for the other way round (12BH7 on L&R, 12AU7 on C)?